1. Field of the Invention
The present invention relates to an electric circuit for cutting-off an output signal when a voltage to be monitored becomes lower than a predetermined level.
2. Description of Related Art
An example of a circuit for cutting-off an output signal is disclosed in JP-A-2003-304633. The circuit is for detecting an open-circuit failure. An essential portion of the circuit is shown in FIG. 5 attached hereto. A signal line 112 of a function circuit 101 is connected to a power source line 141 through a resistor 121 and to a ground line 113 through a resistor 122. The power source line 111 is connected to the ground line 113 through a resistor 123.
The power source line 111 of the function circuit 101 is connected to a power source line 141 of a host circuit 104 that supplies power to the function circuit 101. The signal line 112 of the function circuit 101 is connected to an output signal terminal 145 through a signal line 142 of the host circuit 104. The function circuit 101 includes a signal generating circuit 103. Signals from an amplifier 131 in the signal generating circuit 103 are outputted from the output signal terminal 145 through the signal lines 112, 142. The ground line 113 of the function circuit 101 is connected to a ground line 143 of the host circuit 104. R01, R02 and R03 show internal resistances in the signal generating circuit 103.
In the host circuit 104, the power source line 141 is connected to the signal line 142 through an internal resistance 124, and the signal line 142 is connected to the ground line 143 through an internal resistance 125. The power source line 141 and the ground line 143 of the host circuit 104 are connected to a power source terminal 144 and a ground terminal 146, respectively.
When the circuit for detecting an open-circuit failure described above is applied to a sensor for an automobile, such as a semi-conductor pressure sensor, a voltage of 5 volts is supplied to the function circuit 101 as a power source voltage VCC. When the circuit is functioning without an open-circuit failure, an output signal voltage of the function circuit 101 varies in a range of 0.3-4.8 volts. If the power supply to the signal generating circuit 103 is discontinued because of an open-circuit in the power source lines in the function circuit 101 and the host circuit 104, the output signal voltage Vout is cut-off by a pull-down resistance that connects the signal lines 112, 142 to the ground lines 113, 143. The output signal voltage Vout becomes to a ground voltage level which is lower than an normal output signal voltage. In this manner, the circuit proposed by JP-A-2003-304633 prevents the output signal voltage from becoming unstable when the open-circuit failure occurs.
In the proposed circuit, the output signal voltage Vout is cut-off by connecting pull-down resistances to the signal lines 112 and 142. However, the following problems are involved in the proposed circuit. First, an output capacity of the amplifier 131 in the signal generating circuit 103 has to be increased because a current always flows through the pull-down resistances. This results in higher power consumption in detecting the open-circuit failure. Secondly, the output signal voltage Vout may not decrease to the ground level immediately after an open-circuit failure or a decrease in the power source voltage VCC occurs. The delay is caused by charges stored in capacitors connected to the signal lines 112, 142 and to the power source lines 111, 141, though these capacitors are not shown in FIG. 5.